1. Field of the Invention
The present invention relates to copolymers of ethylene and butadiene, to a catalytic system usable for the synthesis of these copolymers and to a process for synthesising these copolymers by means of this catalytic system.
2. Description of Related Art
It is known to use catalytic systems based on halogenated complexes of transition metals such as titanium for copolymerising ethylene and a conjugated diene.
Japanese patent specifications JP-A-10 237 131, JP-A-09 316 118 and JP-A-11 171 930 disclose copolymers of ethylene and butadiene in which the butadiene may be inserted in the form of cis-1,4, trans-1,4, -1,2, cis-cyclopentyl and trans-cyclopentyl linkages. These copolymers are obtained by means of a catalytic system comprising dimethylsilyl(pentamethylcyclopentadienyl)(t-butylamide)titanium dichloride and methylalumoxane.
It is also known to use catalytic systems based on halogenated lanthanide complexes in order to copolymerise ethylene and a conjugated diene.
In particular, patent specification EP-A-1 092 731 and the article “Macromolecules, vol. 33, no. 23, pp. 8521–8523 (2000)” teach in their copolymerization tests that associating a co-catalyst selected from among a group comprising an alkylmagnesium, an alkyllithium, an alkylaluminium, a Grignard reagent or a mixture of these constituents, with an organometallic complex represented by one of the following generic formulae A or B:
                where Ln represents a lanthanide metal having an atomic number which may range from 57 to 71,        where X represents a halogen which may be chlorine, fluorine, bromine or iodine,        where, in the formula A, two identical or different ligand molecules Cp1, Cp2, each consisting of a substituted or unsubstituted cyclopentadienyl or fluorenyl group, are attached to Ln        where, in the formula B, a ligand molecule consisting of two cyclopentadienyl or fluorenyl groups Cp1 and Cp2, which are substituted or unsubstituted and are linked together by a bridge P satisfying the formula MR2, where M is an element of column IVa of Mendeleyev's periodic table and where R is an alkyl group with 1 to 20 carbon atoms, is attached to said metal Ln,        where the (co-catalyst/organometallic complex) ratio is equal to 20, makes it possible to synthesize copolymers of ethylene and a conjugated diene with a satisfactory polymerisation yield.        
In said document EP-A-1 092 731, it would seem that the copolymers obtained comprise units resulting from the butadiene which, depending upon the catalytic system used, may or may not comprise linkages in the form of trans-1,2 cyclohexane. Furthermore, the copolymers obtained in said document exhibit, when they have a molar content of units resulting from butadiene of at least 8%, a number-average molecular mass Mn, measured by the polystyrene equivalent size exclusion chromatography technique (SEC technique described in the attached appendix), which is low, always being distinctly below 40,000 g/mol.